It was important to both teams, and the Canadiens and Leafs rose to the occasion.
Max Pacioretty and Rene Bourque staked the Canadiens to a 2-0 lead early in the first period.
Joffrey Lupul and Tyler Bozak tied it up, but Brian Gionta beat James Reimer with 53 seconds left in the period.
After a scoreless second period, Nazem Kadri scored on a power play to make it 3-3 before Tomas Plekanec, who was in the pnalty box for Kadri's goal, bagged the winner.
Shots were 36-36.
The win moved the Canadiens five points ahead of Toronto in Atlantic Division standings.
Tomas Plekanec was in the penalty box when Nazem Kadri completed a neat passing play to tie the game at 3-3.
Then on a Canadiens power play, Plekanec converted Andrei Markov's feed for the winning goal.
It was his first in 23 games.
CH survives surge
Toronto outshot the Canadiens 13-8 ... and that doesn't include the posts clanked by Jake Gardiner and Morgan Reilly.
Should be a terrific third period.
Last shot wins
The Canadiens grabbed an early lead on Max Pacioretty's 31st goal and RENE BOURQUE's ninth.
A Joffrey Lupul one-timer gave the Leafs life, and Tyler Bozak tied it.
The deadlock lasted 64 seconds.
With less than a minute left in the period, some yeoman board work by BOURQUE and Daniel Briere popped the puck loose to Brian Gionta, who scored his 15th.
Canadiens outshot Toronto 14-13.
Your basic big one
Your Montreal Canadiens hook up with the Leafs in a classic four-pointer at the ACC.
The Canadiens are third in the Atlantic Division, three points ahead of the Leafs, who hold a wild-card playoff spot. The race tightened when Detroit won on Saturday afternoon.
This is the last of five meetings between the Highway 401 rivals.
The Canadiens have won two of the three games played at the Bell Centre, while the Leafs won the teams' Jan. 18 game at the ACC.
Key lineup move: Alex Galchenyuk will centre the Canadiens' third line, with Brian Gionta and Rene Bourque on his wings. This is the kid's chance to shine in an important game.
Puck drops 7:10ish, after the Hockey Night in Canada studio crew are finished waving their blue and white pom-poms.
Check back later for some bleu-blanc-rouge live blogging.